This invention relates to additives for cement compositions. More particularly, this invention relates to corrosion inhibiting accelerators for hydraulic cements which combine a corrosion inhibitor with a graft copolymer plasticizer, and to cement compositions which use these additives to inhibit the corrosion of metal in contact with the cement.
Alkali and alkaline earth metal nitrites are well known as corrosion inhibiting additives for hydraulic cements. U.S. Pat. No. 3,427,175, for example, relates to Portland cement compositions containing from about 0.1 to 10 percent calcium nitrite based on the dry weight of Portland cement, and discloses that calcium nitrite inhibits corrosion of iron and steel reinforcing structures in the cement. Calcium nitrite is generally preferred over other alkali and alkaline earth metal nitrites, e.g. sodium nitrite, because it provides effective corrosion inhibition without a reduction in compressive strength or other deleterious effects.
When alkali or alkaline earth metal nitrites are used in hydraulic cement compositions in the concentrations generally necessary to achieve substantial corrosion inhibition, e.g. greater than 0.5% by weight of nitrite based on the weight of dry cement, the nitrite normally also provides a substantial level of set acceleration. This acceleration is advantageous, as unaccelerated cements generally require extended setting times, creating delays for the user. In some applications the level of set acceleration prvided by the calcium nitrite alone may be advantageous, (e.g. under cool conditions), while in other applications a slightly reduced level of acceleration may be more desirable (e.g. in warm weather applications). The addition of the nitrite also tends to reduce the slump, or plasticity, of the cement compositions such that the fluidity of the composition is lower initially, and is retained for a shorter time, making the composition more difficult to handle and allowing less working time in the interval prior to setting.
In the past, attempts have been made to increase the slump of cement compositions containing alkali or alkaline earth metal nitrites, and to increase the period of time during which these compositions retain their fluidity. One appraoch has been to add separately to the cement composition a plasticizer which is capable of increasing the initial slump and extending to a degree the period of time during which the composition remains fluid. However, a high level of the plasticizer is generally required in order to obtain these effects, and the addition of this large amount of plasticizer causes excessive retardation of the composition and significantly increases its cost. This practice further requires the cement producer or user to purchase, store and dispense two additives, with added inconvenience, labor and cost, and an increased opportunity for error or miscalculation in dispensing the proper respective amounts of the additives.
It has also been desired in some applications, as described above, to control the amount of acceleration provided by the nitrite, such that the setting time of the cement composition falls somewhere between that of an unaccelerated composition and that obtained when the nitrite alone is added to the composition. One approach used to obtain this effect has been to introduce to the composition an additive comprising the nitrite, water, and certain set retarders which form a soluble admixture with the water and nitrite, e.g. hydroxycarboxylic acids, alkali metal or alkaline earth metal salts thereof, or corn syrup, as disclosed in U.S. Pat. No. 4,466,834. This approach, while providing advantageous setting times, does not provide compositions having high initial slump without causing an undesirable level of retardation, nor does it extend the period of time during which the compositions remain fluid.
Thus an additive is desird which, when added to hydraulic cement compositions, will provide a desired level of corrosion inhibition and set acceleration, but will nonetheless provide a high level of initial fluidity to the cement slurry and a high level of retention of this fluidity for a desired period of time prior to the accelerated setting of the composition.